Add tests for async-iterating a ReadableStream. See whatwg/streams#980 for the accompanying spec change.

This PR continues from @devsnek's work in #13362.

The tests are based on Domenic's comment in whatwg/streams#954. So far, the following tests have been implemented:

• Basic test of a stream with a few chunks that closes
  • Using a "push" underlying source (calls c.enqueue() in start) that enqueues all at once
  • Using a "push" underlying source where you call c.enqueue() "just in time" (i.e. inside the loop body)
  • Using a "pull" underlying source (calls c.enqueue() in pull) using recordingReadableStream to verify the correct underlying source method calls
• Degenerate streams
  • Async-iterating an errored stream throws
  • Async-iterating a closed stream never executes the loop body, but works fine
  • Async-iterating an empty but not closed/errored stream never executes the loop body and stalls the async function (end the test if 50 ms pass without the promise resolving)
• @@asyncIterator() method is === to getIterator() method
• Cancellation behavior (use recordingReadableStream)
  • Manually calling return() causes a cancel
  • throwing inside the loop body causes a cancel
  • breaking inside the loop body causes a cancel
  • returning inside the loop body causes a cancel
  • All of the above, but with preventCancel: true and the pass conditions reversed
• Manual manipulation
  • double-return rejects (#954 (comment))
  • next()'s fulfillment values have exactly the right shape (Object.prototype, only two properties, correct property descriptors)
  • throw method does not exist
  • Calling next() or return() on non-ReadableStreamDefaultReaderAsyncIterator instances rejects with TypeError (there are existing brand check tests you can add to)
  • Calling return() while there are pending reads (i.e. unsettled promises returned by next()) rejects the return Promise (#950 (comment))
• Interaction with existing infrastructure
  • getIterator/@@asynciterator throw if there's already a lock
  • Monkey-patch getReader and default reader's prototype methods and ensure this does not interfere with async iteration (e.g. using one of the basic test cases)
  • Basic test with a few chunks but you've already consumed one or two via a normal reader (which you then released)
• Auto-release
  • You can still acquire a reader and successfully use its .closed promise after exhausting the async iterator via for-await-of
  • You can still acquire a reader and successfully use its .closed promise after return()ing from the async iterator (either manually or via break; take your pick)